It is known to use radiation scanning methods for the monitoring of large structures and vessels such as process vessels and storage tanks. In a typical scanning operation of a process vessel, a radioactive source located adjacent the exterior surface of the vessel is arranged to emit radiation through the vessel and a radiation detector is located adjacent a different portion of the vessel to detect radiation that has passed through the vessel. The amount of radiation passing through the vessel is an indication of the density of the material. In this way the level of the material or of more than one material within the vessel may be monitored. The method is also useful for detecting the presence of process equipment within the vessel, so that, for example, the presence of trays in a distillation column can be verified.
The method used typically requires the radiation source and the detector to be aligned along a linear path between the source and detector and moved to different locations outside the vessel whilst remaining in alignment with each other. This procedure normally requires at least two skilled operators to be present, usually at the top of the vessel. Such a procedure is therefore labour intensive and may be hazardous to the operators, particularly in bad weather. It is an object of the present invention to provide a method of scanning a structure which overcomes some of these difficulties.